One known method for forming such a through aperture uses a punch which has an outer cylindrical surface terminating in a cutting edge, the punch being displaced in the direction of the generatrices of its cylindrical surface with respect to the tubular wall, so that the cutting edge comes into contact With the latter on the outside and passes through its thickness so as to form the aperture, the peripheral surface of the latter being entirely in contact with the cylindrical surface of the punch at the end of the forward stroke of the punch.
Such a method produces a through aperture having a cylindrical peripheral surface joined to the outer surface of the tubular wall, in the portions which are furthest away from the axial plane of the tubular wall parallel to the generatrices of the cylindrical surface of the punch, with an acute angle being defined between the cylindrical peripheral wall of the aperture and the cylindrical outer surface of the tubular wall. This acute junction angle defines a cutting edge, which tends to make a nick in the end of the tube when the latter is being inserted into the aperture if it is not perfectly aligned with the aperture. This danger is increased according to the number of tubes to be fitted simultaneously, being greater the greater the number of tubes.